But... the odds would be against you, ken. White oaks are pretty compatible across their section, but the red/black group...not really reliably compatible, even on their own seedlings, and not at all on members of the white oak group. But... with some genera - or even families - you can get by...I've successfully grafted apple onto pear(not long lived, but successful for a few years), pear onto hawthorn (15 years and counting), plum and peach onto P.tomentosa(Nanking cherry), and quince has historically been used as a dwarfing understock for pear and medlar. Googling up 'shower tree'...it's Cassia fistula...not graft compatible with mango or avocado, so far as I'm aware...

Looks like fungus or disease attacked the branch starting at the wound hole. In the best case it might be possible to save the branch if the cracked bark/dead area is removed along with any soft/decayed wood. Then you could regularly paint on liquid copper fungicide (including under the sides of the rod and on the other side of the rod) until the tree grows around from each side and closes its wound. It would take several years however and no guarantees of full recovery. The next best case you would have to add a wood post to support that corner of the tree house and cut that one branch off below any bit of dead area to prevent disease from reaching the trunk, then regularly paint on liquid copper fungicide until the cut branch closes up the wound over a few years. The worst case is if the tree is deeply infected with any bad disease at that branch, it could spread all the way down to the trunk eventually and kill the whole tree over several years.

Pip your Carmen looks great. Mine seems to have had a lot of leaves broken. Was my first time ordering online. Four winds is good for hard to find varieties. Unfortunately finding harder to find varieties in the shops is difficult. Drove around way too much. Hopefully the tree grows vigorously.

In the past when my grapefruit has dropped leaves, it was water DEFICIENT. This was difficult to figure out because when I would try watering it, more leaves would fall off - making me think it may have been TOO MUCH water and by watering it I made it worse! But I've learned that there is a delayed reaction with this tree. It DID need water, but by giving it the water it needs, it wanted to drop the half-dead leaves so it could grow new ones. Because immediately after the leaves fell off new leaves started growing. In fact one year it bloomed and I actually got 2 grapefruits. (And I grew this in Syracuse from a seed I planted from a store-bought fruit!)